A mouse anti-interleukin-2 receptor A-chain-specific PC61-immunotoxin (PC61-IT) strongly inhibited a primary mixed lymphocyte culture and major histocompatibility complex (MHC)-restricted cytotoxicity. The allodepleted T cells retained their proliferative and cytotoxic capacities in response to third-party stimulation, showing that PC61-IT specifically deleted recipient antigen-specific T-cell clones from the donor mouse. The ability of this specific allodepletion to prevent graft-versus-host disease (GVHD) and graft rejection was investigated in vivo. IT-depleted, activated parental T lymphocytes (C3H/eB) were intravenously injected into lethally irradiated CDF1 mice. GVHD was evaluated after 6 days on the severity of gut lesions. PC61-IT-treated cells significantly reduced both donor T-cell infiltration and acceleration of epithelial renewal (a sensitive index of gut damage) as compared with those for the corresponding untreated controls. The effect of selective allo-depletion on prevention of GVHD and graft rejection was further studied after MHC-haploincompatible bone marrow (BM) transplantation. A significant increase in survival was observed in mice receiving 2 x 10(6) T-cell-depleted BM cells and 0.5 x 10(6) PC61-IT-treated T cells, because one-third were alive without GVHD (and with stable full or partial engraftment) after 100 days, whereas all the mice infused with BM and sham-treated T cells died within 80 days from GVHD, and all the mice infused with BM cells alone rejected grafts. Furthermore, specific tolerance in chimeras towards donor cells could be shown. These results as observed in an experimental in vivo model corroborate previous results obtained in vitro in humans and lead us to consider the use of this selective allodepletion in human BM transplant from donors other than identical familial siblings.